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//! [`Color`]: straight-alpha sRGB with `f32` channels, plus the small
//! amount of colour arithmetic a palette needs (mixing, luminance, WCAG
//! contrast).
/// A straight-alpha sRGB colour with `f32` channels in `0.0..=1.0`.
///
/// Build one from bytes, a hex literal or floats, and derive the rest of a
/// palette with [`Color::mix`] and [`Color::with_alpha`]:
///
/// ```rust
/// use kui_core::Color;
///
/// let brand = Color::hex(0x3b5bd4ff);
/// assert_eq!(brand, Color::rgb8(0x3b, 0x5b, 0xd4));
/// let hover = brand.mix(Color::WHITE, 0.1); // a little lighter
/// let wash = brand.with_alpha(0.25);
/// assert!(hover.r > brand.r && wash.a == 0.25);
/// assert_eq!(brand.to_hex(), 0x3b5bd4ff);
/// ```
#[repr(C)]
#[derive(Clone, Copy, Debug, Default, PartialEq)]
pub struct Color {
pub r: f32,
pub g: f32,
pub b: f32,
pub a: f32,
}
impl Color {
pub const TRANSPARENT: Color = Color {
r: 0.0,
g: 0.0,
b: 0.0,
a: 0.0,
};
pub const WHITE: Color = Color {
r: 1.0,
g: 1.0,
b: 1.0,
a: 1.0,
};
pub const BLACK: Color = Color {
r: 0.0,
g: 0.0,
b: 0.0,
a: 1.0,
};
pub fn rgba(r: f32, g: f32, b: f32, a: f32) -> Self {
Self { r, g, b, a }
}
pub fn rgb8(r: u8, g: u8, b: u8) -> Self {
Self::rgba8(r, g, b, 255)
}
pub fn rgba8(r: u8, g: u8, b: u8, a: u8) -> Self {
Self {
r: r as f32 / 255.0,
g: g as f32 / 255.0,
b: b as f32 / 255.0,
a: a as f32 / 255.0,
}
}
/// This colour as `0xRRGGBBAA`, rounded to eight bits a channel: what
/// [`Color::hex`] reads.
pub fn to_hex(&self) -> u32 {
let ch = |v: f32| (v.clamp(0.0, 1.0) * 255.0).round() as u32;
(ch(self.r) << 24) | (ch(self.g) << 16) | (ch(self.b) << 8) | ch(self.a)
}
/// A colour from `0xRRGGBBAA`.
pub fn hex(v: u32) -> Self {
Self::rgba8((v >> 24) as u8, (v >> 16) as u8, (v >> 8) as u8, v as u8)
}
pub fn with_alpha(mut self, a: f32) -> Self {
self.a = a;
self
}
pub fn is_visible(&self) -> bool {
self.a > 0.0
}
/// The colour as a tween's four lanes.
#[inline]
pub(crate) fn lanes(self) -> [f32; 4] {
[self.r, self.g, self.b, self.a]
}
/// The inverse of [`Self::lanes`].
#[inline]
pub(crate) fn from_lanes(v: [f32; 4]) -> Color {
Color {
r: v[0],
g: v[1],
b: v[2],
a: v[3],
}
}
/// Every channel, alpha included, `t` of the way to `other` — what a
/// tween does to a colour. [`Self::mix`] keeps this colour's alpha.
pub fn lerp(self, other: Color, t: f32) -> Color {
Color::from_lanes(std::array::from_fn(|i| {
self.lanes()[i] + (other.lanes()[i] - self.lanes()[i]) * t
}))
}
/// This colour moved `t` of the way toward `other`, per channel, with
/// `self`'s alpha kept: `mix(WHITE, 0.1)` is "a little lighter" and
/// `mix(BLACK, 0.1)` "a little darker", which is how a palette is
/// built from one colour (`widgets::button_palette`).
///
/// Straight sRGB, not a perceptual space, and the same interpolation a
/// colour transition does. `t` outside `0..=1` extrapolates rather than
/// clamping, so a caller can overshoot on purpose.
pub fn mix(self, other: Color, t: f32) -> Color {
Color {
r: self.r + (other.r - self.r) * t,
g: self.g + (other.g - self.g) * t,
b: self.b + (other.b - self.b) * t,
a: self.a,
}
}
/// WCAG relative luminance, 0 (black) to 1 (white): the number a
/// contrast decision is made from — "is this background light enough
/// to want dark text" — rather than the average of the channels, which
/// would call a saturated blue and a saturated green equally bright.
/// Alpha is not in it; a translucent colour's luminance is the
/// luminance of what it would be over nothing.
pub fn luminance(self) -> f32 {
let ch = |c: f32| {
let c = c.clamp(0.0, 1.0);
if c <= 0.03928 {
c / 12.92
} else {
((c + 0.055) / 1.055).powf(2.4)
}
};
0.2126 * ch(self.r) + 0.7152 * ch(self.g) + 0.0722 * ch(self.b)
}
/// WCAG's contrast ratio between two opaque colours, 1:1 to 21:1 —
/// the number "4.5:1" (body text) and "3:1" (large text, UI edges, a
/// focus ring) are ratios of. Symmetric. The arithmetic a palette is
/// *checked* with; which pairs to check is the theme's business.
pub fn contrast(self, other: Color) -> f32 {
let (a, b) = (self.luminance(), other.luminance());
(a.max(b) + 0.05) / (a.min(b) + 0.05)
}
/// `self` moved toward `toward` in steps of 0.05, starting `from` of
/// the way there, until it clears `ratio` on `on` — the one loop
/// behind "an accent that can be seen on this base". Reaches
/// `toward` itself at the end, so pick one that clears the ratio on
/// its own (black or white on any base does), and the cap is a floor
/// rather than a give-up. A colour that already reads at `from` comes
/// back as that mix, so `from = 0.0` paints one verbatim.
pub fn toward_contrast(self, toward: Color, on: Color, ratio: f32, from: f32) -> Color {
let mut t = from.clamp(0.0, 1.0);
loop {
let c = self.mix(toward, t);
if t >= 1.0 || c.contrast(on) >= ratio {
return c;
}
t = (t + 0.05).min(1.0);
}
}
}
#[cfg(test)]
mod tests {
use super::*;
/// A mix keeps its own alpha, lands on each end at 0 and 1, and is
/// the arithmetic a palette is built with.
#[test]
fn a_mix_walks_between_two_colors() {
let a = Color::rgba(0.2, 0.4, 0.6, 0.5);
assert_eq!(a.mix(Color::WHITE, 0.0), a);
let end = a.mix(Color::WHITE, 1.0);
assert_eq!((end.r, end.g, end.b), (1.0, 1.0, 1.0));
assert_eq!(end.a, 0.5, "the alpha is this colour's, not the other's");
let half = a.mix(Color::BLACK, 0.5);
assert!((half.r - 0.1).abs() < 1e-6 && (half.b - 0.3).abs() < 1e-6);
}
/// Black on white is WCAG's 21:1, a colour on itself 1:1, and the
/// ratio does not care which side is which.
#[test]
fn contrast_is_wcags_ratio_and_symmetric() {
assert!((Color::BLACK.contrast(Color::WHITE) - 21.0).abs() < 1e-3);
assert!((Color::WHITE.contrast(Color::BLACK) - 21.0).abs() < 1e-3);
let c = Color::hex(0x3b5bd4ff);
assert!((c.contrast(c) - 1.0).abs() < 1e-6);
assert_eq!(c.contrast(Color::WHITE), Color::WHITE.contrast(c));
}
/// The loop stops at the first step that reads, paints a colour that
/// already reads verbatim when started from 0, and ends on `toward`
/// when nothing short of it does.
#[test]
fn toward_contrast_stops_at_the_first_step_that_reads() {
let navy = Color::hex(0x1e2a4aff);
let dark = Color::hex(0x1a1d27ff);
assert!(
navy.contrast(dark) < 1.5,
"the case: an accent near the base"
);
let ink = navy.toward_contrast(Color::WHITE, dark, 3.0, 0.0);
assert!(ink.contrast(dark) >= 3.0);
assert!(
ink.contrast(dark) < 3.5,
"the first step that clears, not a leap"
);
let already = Color::hex(0xf5d67fff);
assert_eq!(
already.toward_contrast(Color::WHITE, dark, 3.0, 0.0),
already
);
assert_eq!(
navy.toward_contrast(Color::WHITE, dark, 30.0, 0.0),
navy.mix(Color::WHITE, 1.0)
);
// `from` is where it starts: a start that already reads is the
// answer, even when a smaller mix would have read too.
let lifted = navy.toward_contrast(Color::WHITE, dark, 3.0, 0.35);
assert_eq!(lifted, navy.mix(Color::WHITE, 0.35));
assert!(ink.r < lifted.r, "from 0 the loop stopped short of 0.35");
}
/// The number a contrast decision is made from: white is 1, black 0,
/// and green outweighs blue at the same channel value — which is the
/// whole reason it is not the average of the three.
#[test]
fn luminance_is_weighted_the_way_eyes_are() {
assert!((Color::WHITE.luminance() - 1.0).abs() < 1e-4);
assert!(Color::BLACK.luminance().abs() < 1e-6);
let green = Color::rgb8(0, 255, 0).luminance();
let blue = Color::rgb8(0, 0, 255).luminance();
assert!(green > blue * 5.0, "{green} vs {blue}");
// The two accents the stock button's split was checked against.
assert!(Color::hex(0x007affff).luminance() < 0.4, "macOS blue");
assert!(Color::hex(0xffc409ff).luminance() > 0.4, "macOS yellow");
}
}